Wireless systems typically up-convert a baseband signal to a Radio Frequency (RF) for transmission, and down-convert received RF signals to baseband for processing. Such frequency conversion requires producing a reliable mixing frequency signal, typically referred to as a local oscillator (LO) signal, for use in the RF front-end of a wireless device. Phase-Locked Loops (PLLs) are often used to provide such mixing frequency signals. In some cases, stringent requirements are placed on the mixing frequency signal, such as produced by a PLL.
It is foreseeable that 5G cellular systems will use millimetre waves, where the frequencies currently in discussion range between 15 GHz and 60 GHz. In order to use such 5G system outdoors, a longer cyclic prefix has to be used compared to newly released 60 GHz indoor systems. Such longer cyclic prefixes necessitate a closer sub-carrier spacing in the OFDM modulation. This closer sub-carrier spacing poses stringent phase noise requirements on the output of the PLLs. At the same time, beamforming should be supported to increase the range and capacity of the system, which results in a large number of antenna elements. The signal at each antenna element of a beamforming system will have an individual phase shift that controls the beam and in particular the beam direction. In some implementations, the beam controlling phase shifts are imposed on the oscillator signal generated by the PLL. In any event, accurate phase shifts are required to provide accurate beamforming. It is also desirable to be able to program the frequency of the oscillator signal to enable the wireless device to operate on different frequency channels and in different bands.
Besides the ability to introduce a programmable phase shift and programmable frequency, the local oscillator generation circuitry shall also achieve a low phase noise and other aspects without consuming excessive power.